Dr Ken Grice of Monsanto: "The bottom line is - it works"

The plastic produced in the plant factories is not only biodegradable, it is also suitable for widespread commercial use. It is being produced experimentally in special varieties of GM oilseed rape and cress.

Conventional plastics are made from oil and do not degrade easily.

But the University of Lausanne's Yves Poirer, commenting on the research published in Nature Biotechnology, said: "There is a growing awareness that petroleum is a finite resource and that the indestructibility of plastics can be more of a nuisance than a benefit.

The plant plastic is flexible enough to be widely used

"Synthesis of the materials in crops represents not only an attractive approach to the renewable production of bioplastics, but also an excellent method of increasing the value of crops by adding novel characteristics to plants."

Scientists have long been looking for ways of making plastics that are better for the environment. They have already tried using special strains of bacteria that produce plastic naturally under certain conditions.

But this is a costly process. One kilogram of this plastic would at best cost $3-5, compared with $1 per kilogram for petroleum-derived plastic. Furthermore, the end product is too brittle for most applications.

Oilseed rape could produce the plastic

However, the scientists at Monsanto in the US have managed to produce biodegradable plastic from plants using genetic engineering.

They have done this by inserting four genes from the plastic-producing bacteria into varieties of oilseed rape and cress. This turns the plants into biological factories making plastic that can then be extracted from the plant.

BBC Science's Helen Briggs: "Plants turned into biological factories"

Unlike bacterial plastics, the plant plastic is suitable for commercial use.

Also plastic-producing bacteria have to be fed carbon, in the form of glucose, which has been extracted from a crop. In contrast, plants take carbon directly from the air and so the plastic from the GM crops is likely to be relatively cheap.

However, the yield of plastic in the crops is currently only 3%. This is six times lower than has been managed in other experiments.

Monsanto scientists say the next step is to refine the GM process to make it suitable for high-yield production.

This may be possible but cannot be taken for granted. Research programmes by both Monsanto and Zeneca, investigating other approaches to bioplastics, have ended in failure.

Environmental group Friends of the Earth criticised Monsanto's work as little more than a public relations exercise designed to restore the company's image after unfavourable publicity over ita association with GM food crops.

"First Monsanto said it is going to feed the world, now it appears they are going to help solve our waste problems," a spokesman said.